It is known to use the curtain coating method in the application of thin layers of liquid compositions to moving sheet materials. The conditions to be respected using such a method have been described by R. D. Brown in Journal Of Fluid Mechanics, 10, 1961 (297-305). Although Brown's teachings are known to the skilled in the art, they can be summarized as follows:
i. For a moderate length curtain such as that employed industrially, the value of Qu (wherein Q is the flow per length unit, u is the curtain velocity and T is the surface tension) should be higher than 2T near the line of impingement of the curtain with the moving surface, since otherwise the disturbance of the flow at the point of impingement will break the curtain. PA0 ii. The curtain should be protected from air currents carried along with the moving surface. PA0 iii. The impingement velocity of the curtain should be above a certain value (of the order of 130 cm/sec.) which is apparently independent of the speed of the surface. PA0 iv. A thin curtain of liquid produced by pumping the liquid through a slot suffers from inherent instability near the slot unless the velocity of the curtain everywhere outside the slot is greater than 2T/Q.
Photographic coating compositions were found to have physical characteristics good for the application of curtain coating. The possibility of using the multiple layer technique, known in the field of bead coating, was also confirmed, as described in U.S. Pat. Nos. 3,615,572 and 3,508,947 (incorporated herein by reference).
Certain coating techniques appeared to be of potential significance to manufacturing processes involving the coating of thin layers by offering the possibility of coating at speeds higher than those normally known or practiced in the art.
Some inherent characteristics of the bead coating method, in fact, made the expert evaluate the maximum speed obtainable with this technique as lower than that obtainable with curtain coating and the quality associated with curtain coating to be better than that obtainable with the bead technique.
Improvements on coating alleys capable of drying photographic film or paper materials coated at high speed were made in view of the indicated possibility of coating at speeds higher than normal in the art and studies were made to optimize the coating method to afford high-quality coating results.
In spite of many efforts spent to study and develop curtain coating, there were still quality problems connected therewith because of non-uniform formation of the curtain while leaving the coating apparatus (or hopper).
Various types of hoppers were designed and built-up to improve the quality characteristics of the coated layers. Normally, a hopper includes (1) one or more feeding bodies which cause the liquid composition to pass through one or more slots and (2) slide surfaces for the sliding of the coating composition until it leaves the apparatus to form the curtain. The end portion of the slide body (3) is normally called the lip (see particularly RD 14715, page 19 wherein various types of lips were described).